The chief aims of this proposal are to characterize the sensory encoding properties of the mechanoreceptors of the vertebrate endocardium in detail in quantitative biophysical terms. Its physiological significance is two-fold: (1) To explain the roles of endocardial tissue mechanics and of cellular ionic mechanisms of electrogenesis in the behavior of mechanical threshold, sensitivity to mechanical stress and to strain, directional properties of the endings, and sensory adaptation in this important class of branched, unencapsulated sensory endings; (2) To elucidate the relationship between the variety of morphological types of endings and the characteristic discharge patterns seen in vagal afferent fibers in vivo. Most of the work will be done with in vitro heart preparations of frog, rat and cat. A compliance microprobe principle, developed in work with insect mechanoreceptors, will be adapted for mechanical measurements in the endocardium. Threshold, sensitivity and adaptation will be described mainly in terms of linear transfer functions using sinusoidal mechanical stimulation. Morphological identification of the physiologically characterized endings will be attempted by backfilling with cobalt, nickel or microperoxidases.